linux-ext4 - Re: [PATCH v2 10/18] fsdax: Manage pgmap references at entry insertion and deletion

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Date:   Wed, 21 Sep 2022 19:07:42 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     akpm@...ux-foundation.org, Matthew Wilcox <willy@...radead.org>,
        Jan Kara <jack@...e.cz>, "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>,
        linux-fsdevel@...r.kernel.org, nvdimm@...ts.linux.dev,
        linux-xfs@...r.kernel.org, linux-mm@...ck.org,
        linux-ext4@...r.kernel.org
Subject: Re: [PATCH v2 10/18] fsdax: Manage pgmap references at entry
 insertion and deletion

On Wed, Sep 21, 2022 at 02:38:56PM -0700, Dan Williams wrote:
> Dan Williams wrote:
> > Jason Gunthorpe wrote:
> > > On Thu, Sep 15, 2022 at 08:36:07PM -0700, Dan Williams wrote:
> > > > The percpu_ref in 'struct dev_pagemap' is used to coordinate active
> > > > mappings of device-memory with the device-removal / unbind path. It
> > > > enables the semantic that initiating device-removal (or
> > > > device-driver-unbind) blocks new mapping and DMA attempts, and waits for
> > > > mapping revocation or inflight DMA to complete.
> > > 
> > > This seems strange to me
> > > 
> > > The pagemap should be ref'd as long as the filesystem is mounted over
> > > the dax. The ref should be incrd when the filesystem is mounted and
> > > decrd when it is unmounted.
> > > 
> > > When the filesystem unmounts it should zap all the mappings (actually
> > > I don't think you can even unmount a filesystem while mappings are
> > > open) and wait for all page references to go to zero, then put the
> > > final pagemap back.
> > > 
> > > The rule is nothing can touch page->pgmap while page->refcount == 0,
> > > and if page->refcount != 0 then page->pgmap must be valid, without any
> > > refcounting on the page map itself.
> > > 
> > > So, why do we need pgmap refcounting all over the place? It seems like
> > > it only existed before because of the abuse of the page->refcount?
> > 
> > Recall that this percpu_ref is mirroring the same function as
> > blk_queue_enter() whereby every new request is checking to make sure the
> > device is still alive, or whether it has started exiting.
> > 
> > So pgmap 'live' reference taking in fs/dax.c allows the core to start
> > failing fault requests once device teardown has started. It is a 'block
> > new, and drain old' semantic.

It is weird this email never arrived for me..

I think that is all fine, but it would be much more logically
expressed as a simple 'is pgmap alive' call before doing a new mapping
than mucking with the refcount logic. Such a test could simply
READ_ONCE a bool value in the pgmap struct.

Indeed, you could reasonably put such a liveness test at the moment
every driver takes a 0 refcount struct page and turns it into a 1
refcount struct page.

Jason